It wasn't made very clear in the article, KE. But the retina has rods and cones (light receptotors). Rods are for black and white/night vision, and cones are for color (like A Hard Day's Night and Help!*).

When they dissect an eye and look at a slide of the retina under a microscope, there are more rods than there are cones in a cat's eye, so the cat doesn't see colors well, but sees well in the dark, but in B&W with just a hint of color. The shape of the retina (think of it as a projection screen in a movie theater) and the concentration of these light receptors in different parts of the retina determine the field of vision and focus.